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ABSTRACT: In this paper the thermal and optical properties of the SiO2/GaN synthetic opals are studied by photothermal deflection technique. This technique, used in different configurations, allows
to determine the effective thermal diffusivity and the absorption spectra.
Optical and Quantum Electronics 04/2012; 39(4):305-310. · 0.82 Impact Factor
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M. C. Larciprete,
A. Bosco,
A. Belardini, R. Li Voti,
G. Leahu,
C. Sibilia,
E. Fazio,
R. Ostuni,
M. Bertolotti,
A. Passaseo,
B. Potì,
Z. Del Prete
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ABSTRACT: We studied the second order optical nonlinearity of aluminum nitride films grown by sputtering onto silicon substrates. The crystalline properties of the films were investigated by x-ray diffraction measurements. Preliminary linear optical characterization of the films was carried out by spectrophotometric optical reflectance measurements at different incidence angles; thus the dispersion laws for both ordinary and extraordinary refractive indices were retrieved. Finally, second harmonic generation measurements in reflection mode were performed at a fixed angle from a fundamental beam provided by a picosecond Ti:sapphire laser system at λ = 800 nm. In the experiments a high blue light conversion efficiency was found for samples 1.5 and 2 μm thick, and the second order nonlinear coefficient d33 = 11±1 pm/V was found.
Journal of Applied Physics 07/2006; 100(2):023507-023507-5. · 2.17 Impact Factor
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ABSTRACT: Photothermal depth profiling is usually applied to inhomogeneous materials to localize the optical inhomogeneity or retrieve
the thermal effusivity depth profile by simply monitoring the photothermal signal after the pump beam excitation. In this
paper the different kinds of inverse problems related to photothermal depth profiling are discussed, and the solutions given
by thermal wave backscattering (TWBS) and genetic algorithms (GAs) are compared. Finally, the different performances and limits
of validity on known linear profiles are compared.
International Journal of Thermophysics 10/2005; 26(6):1833-1848. · 0.95 Impact Factor
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ABSTRACT: Photothermal reflectance methods have been intensively applied to the nondestructive testing of opaque thin films [D. P. Almond and P. M. Patel, Photothermal Science and Techniques (Chapman and Hall, London, 1996); C. Bento and D. P. Almond, Meas. Sci. Technol. 6, 1022 (1995); J. Opsal, A. Rosencwaig, and D. Willenborg, Appl. Opt. 22, 3169 (1983)]. The basic principle is based on thermal wave interferometry: the opaque specimen is illuminated by a laser beam, periodically chopped at the frequency f, so as to generate a plane thermal wave in the surface region. This wave propagates in the film, approaches the rear interface (film-bulk), is partially reflected back, reaches the front surface, is again partially reflected back and so on, giving rise to thermal wave interference. A consequence of this interference is that the surface temperature may be enhanced (constructive interference) or reduced (destructive interference) by simply scanning the frequency f (that is, the thermal diffusion length μ= D/πf ), so as to observe damped oscillations as a function of f; in practice only the first oscillation may be clearly resolved and used to measure either the film thickness d or the film thermal diffusivity D, and this situation occurs when μ≈d. In general, photothermal reflectance does not measure directly the surface temperature variation, but rather a directly related signal determined by the thermo-optic coefficients and the sample geometry; for detection it is common to monitor the optical reflectivity variation of a probe beam normally incident on the sample. If the thin film is partially transparent to the probe, the theory becomes more difficult [O. Matsuda and O. B. Wright, J. Opt. Soc. Am. B (in press)] and one should consider the probe beam multiple reflections in the thin film. The probe modulation is optically inhomogeneous due to the temperature-induced-
changes in refractive index. Although in the past the complexity of the analysis has impeded research in this field, we show how a general analytical method can be used to deal with photothermal reflectance data for transparent thin films. We apply this method to a thin film of silica on a silicon substrate [O. B. Wright, R. Li Voti, O. Matsuda, M. C. Larciprete, C. Sibilia, and M. Bertolotti, J. Appl. Phys. 91 5002 (2002)]. © 2003 American Institute of Physics.
Review of Scientific Instruments 02/2003; · 1.37 Impact Factor
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ABSTRACT: Photothermal depth profiling has been the subject of many papers in the last years. Inverse problems on different kinds of materials have been identified, classified, and solved. A first classification has been done according to the type of depth profile: the physical quantity to be reconstructed is the optical absorption in the problems of type I, the thermal effusivity for type II, and both of them for type III. Another classification may be done depending on the time scale of the pump beam heating (frequency scan, time scan), or on its geometrical symmetry (one- or three-dimensional). In this work we want to discuss two different approaches, the genetic algorithms (GA) [R. Li Voti, C. Melchiorri, C. Sibilia, and M. Bertolotti, Anal. Sci. 17, 410 (2001); R. Li Voti, Proceedings, IV Int. Workshop on Advances in Signal Processing for Non-Destructive Evaluation of Materials, Quebec, August 2001] and the thermal wave backscattering (TWBS) [R. Li Voti, G. L. Liakhou, S. Paoloni, C. Sibilia, and M. Bertolotti, Anal. Sci. 17, 414 (2001); J. C. Krapez and R. Li Voti, Anal. Sci. 17, 417 (2001)], showing their performances and limits of validity for several kinds of photothermal depth profiling problems: The two approaches are based on different mechanisms and exhibit obviously different features. GA may be implemented on the exact heat diffusion equation as follows: one chromosome is associated to each profile. The genetic evolution of the chromosome allows one to find better and better profiles, eventually converging towards the solution of the inverse problem. The main advantage is that GA may be applied to any arbitrary profile, but several disadvantages exist; for example, the complexity of the algorithm, the slow convergence, and consequently the computer time consumed. On the contrary, TWBS uses a simplified theoretical model of heat diffusion in inhomogeneous materials. According to such a model, the photothermal signal depends linearly on the thermal effusivity inhomogeneities, which may be detected because they act as backscattering centers for the heat flux. The physical problem is reduced to the inversion of a algebraic linear system. The advantage is that TWBS allows excellent reconstructions, but only within the limits of validity of the approximate model, which include any slowly varying profile. Recently we have tested the perfomance of both TWBS and GA on linear conductivity profiles. In other words, we have done the numerical simulations of the photothermal measurements coming from a film over a substrate, where the conductivity in the film changes linearly from k1 at the surface, to k2 at the substrate. TWBS and GA have been used to reconstruct the original profiles. If the conductivity mismatch ranges as 0.2<k1/k2<5, the error averaged over the whole profile is lower than 1% for TWBS, and lower than 2% for GA. However, in the case of a stronger conductivity mismatch GA exhibits better performances. These results will be widely discussed in a publication in the near future. © 2003 American Institute of Physics.
Review of Scientific Instruments 12/2002; 74(1):372-372. · 1.37 Impact Factor
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ABSTRACT: We investigate thermally induced transmission variations in a 3.6-μm-thick ZnSe/MgF2 photonic band gap structure by means of a pump–probe setup, in the 600–700 nm range, under cw pump conditions. An induced temperature increase is responsible for the thermal expansion of the layers, as well as changes in the index of refraction. As a result, the band gap structure is redshifted by several nanometers. The initial transmission of the probe beam was restored following the removal of the pump laser, thus indicating the reversible nature of the process. © 2002 American Institute of Physics.
Journal of Applied Physics. 08/2002; 92(5):2251-2255.
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ABSTRACT: The accuracy of photothermal depth profiling of the thermal conductivity profile of thermally inhomogeneous materials is discussed in detail. The reconstruction error is statistically analysed as a function of depth and noise, for different classes of profiles. The analysis is done in the framework of reconstructions obtained by a neural network approach, thermal wave backscattering theory and a method which is based on a statistical sampling of the chi-squared function in a restricted part of the profile space. The latter analysis yields confidence intervals for the reconstructions.
Inverse Problems 10/1999; 15(5):1149. · 1.88 Impact Factor
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ABSTRACT: This article describes and proves the basic phenomena which take place when thermal waves approach an interface between two media: the reflection and the refraction. In synthesis the Snell law for plane thermal waves is proved, both theoretically and experimentally, by means of the mirage technique. © 1999 American Institute of Physics.
Journal of Applied Physics 05/1999; · 2.17 Impact Factor
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ABSTRACT: In this paper we want to introduce the theory of depth profiling in time domain. We present a new theoretical model for the heat diffusion in media with variable thermal parameters and show the retrieval procedure to reconstruct the thermal effusivity depth profile from the surface temperature dynamic. The effect of the noise on the quality of the reconstruction is also briefly discussed. © 1999 American Institute of Physics.
AIP Conference Proceedings. 03/1999; 463(1):37-39.
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ABSTRACT: In this paper we want to discuss several new aspects in the theory of depth profiling, introducing a new theoretical model for the heat diffusion in media with variable thermal parameters. The case of thermal conductivity and diffusivity, both are functions of depth is widely analyzed. The thermal depth profiles, reconstructed by numerical simulations according to this new model, are in excellent agreement with the original ones. Experimental results for steel samples hardened at the surface are also presented. © 1999 American Institute of Physics.
AIP Conference Proceedings. 03/1999; 463(1):24-26.
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ABSTRACT: The photothermal deflection technique has been used to determine the thermophysical properties of artificially aged paper samples. The deflection amplitude ratio and the phase difference of the deflection angle between front and rear surface illumination have been measured as a function of the pump beam modulation frequency, in order to avoid the dispersion effects mainly introduced by the probe beam vertical offset with respect to the sample surface. A theoretical model for the photothermal signal generation, taking into account the unavoidable light scattering occurring at the paper fibers–air interfaces, has been developed. A photothermal deflection method has been also proposed to measure the transmittance and the reflectance values of a given paper sheet, thus offering a proper constraint to the whole fitting procedure. In this way it has been possible to estimate the thermal diffusivity, the optical absorption and the scattering coefficient values of all the samples examined. A correlation between these values and the aging state of the paper has been found. © 1999 American Institute of Physics.
Journal of Applied Physics 02/1999; 85(5):2881-2887. · 2.17 Impact Factor
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ABSTRACT: The thermal response of a semiconductor laser diode is studied through two different methods based on the photothermal deflection technique. The full theoretical model is reviewed; the study of the temperature profile and hence the maximum mirror temperature rise, obtained for different sizes of the device, allow one to expect thermally stable lasers. The way to obtain, from the measurements, the thermal parameters of the entire structure (diffusivity, conductivity) is also considered. The photothermal deflection equipment is described in detail for this application. The experimental results on three different kinds of laser diode are thoroughly discussed: double-heterostructure AlGaAs/GaAs, double-heterostructure InGaAsP/InP, and single-quantum-well (QW) structure AlGaAs/GaAs InGaAs QW.
Measurement Science and Technology 12/1998; 6(9):1278. · 1.49 Impact Factor
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ABSTRACT: The photothermal deflection technique has been usually applied, for the thermal diffusivity measurements, in the transverse skimming scheme. To overcome some limitations of the skimming, a surface reflection scheme (i.e., bouncing scheme) has been introduced in which the probe beam is reflected from the sample surface. In this configuration the probe beam deflection is obtained as a result of two different mechanisms: the thermal gradient in the gas near to the heated sample (mirage) and the sample surface deformation due to the thermal expansion (displacement). The superposition of these two effects must be taken into account when deriving the thermal diffusivity. In this article the mirage and the displacement have been studied from a theoretical and experimental point of view, and a new method for the measurement of thermal diffusivity in the bouncing scheme is presented. A special setup is described to obtain separately the mirage and the displacement signals from which the thermal diffusivity and the thermal expansion coefficient can be derived. The experimental values for different samples obtained by applying our method are in agreement with the literature values. © 1998 American Institute of Physics.
Journal of Applied Physics 02/1998; · 2.17 Impact Factor
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Applied Physics B 01/1998; 67(5):641-646. · 2.19 Impact Factor
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ABSTRACT: The photothermal deflection technique is applied in transverse configuration to measure the thermal diffusivity of semiconductor wafers. The large size of these samples inhibits the possibility to make the probe beam skim the sample at a small height which is required for a direct thermal diffusivity measurement. To overcome this problem, three new experimental schemes are proposed, each one based on a different geometry of the heat diffusion (one-, two-, or three-dimensional scheme). In particular for the 3D experimental scheme, a new mirage setup is described which uses two crystalline prisms 6 mm apart from each other to let the probe beam skim 50±3 μm high over the sample surface, with a spot size of 22 μm. The main advantages of this setup, here discussed, are the obtained low probe beam height which is, moreover, independent of the sample dimensions, and the cheap technology to produce the necessary high-quality prisms. The performances of the new schemes have been tested by comparing, for well-known semiconductor wafers (InSb, InAs, InP, GaAs, GaP, Ge, and Si), the experimentally measured thermal diffusivity with the values reported in the literature. © 1997 American Institute of Physics.
Review of Scientific Instruments 02/1997; 68(3):1521-1526. · 1.37 Impact Factor
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ABSTRACT: Summary form only given. The optical beam deflection technique has been applied to the optothermal characterization of solids. The technique is nondestructive and contactless, giving the opportunity to work in hostile environments. It has been proved to be very sensitive to small temperature rises (10/sup -3/) and it is applicable to a wide set of samples with different geometries.
Lasers and Electro-Optics, 1996. CLEO '96., Summaries of papers presented at the Conference on; 07/1996
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ABSTRACT: Summary form only given. During the past few years a great deal of attention has been focused on the development of photothermal methods in order to characterize the thermal properties of solid samples. In this work the harmonic local surface deformation induced by a Gaussian pump laser beam modulated in time is studied through the photothermal method by the direction change of a second laser beam (probe) reflected by the bended surface.
Lasers and Electro-Optics, 1996. CLEO '96., Summaries of papers presented at the Conference on; 07/1996
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ABSTRACT: The photodeflection technique is useful not only for thermal diffusivity measurements but also to supply a thermal imaging system. The experimental setup and the basic theoretical aspects for determining the temperature profile are discussed together with the experimental results on a semiconductor laser diode.
Journal of Thermal Analysis and Calorimetry 06/1996; 47(1):51-65. · 1.60 Impact Factor
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ABSTRACT: A cryostatic setup is described to perform photothermal deflection measurements from room temperature to 77 K. The setup uses gaseous nitrogen as a medium where the photodeflection is produced. The ability of the system to work is demonstrated presenting some measurements of thermal diffusivity of high‐temperature superconductor samples and of yttrium‐iron garnets with variable aluminum content. © 1995 American Institute of Physics.
Review of Scientific Instruments 01/1996; · 1.37 Impact Factor
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ABSTRACT: Thermal diffusivity for carbon fiber composites was measured using different fiber types and polymer matrices. Photothermal testing was performed in the various directions parallel and perpendicular to the carbon fiber axis by different photothermal configurations. By focusing the laser beam with a spherical lens, local inhomogeneities of the composite surface in the range of 10 μm are distinguished. When focusing is done with the aid of a cylindrical lens an averaging over larger scales of the photothermal deflected signal takes place. The results for various carbon fiber materials are discussed in terms of thermal diffusion lengths and thermal diffusivity values. It is shown that the thermal photodeflection method is suitable for measuring anisotropy in oriented carbon fiber composites. © 1995 American Institute of Physics.
Journal of Applied Physics 12/1995; · 2.17 Impact Factor
